The present invention relates to an improved process for preparing TiO.sub.2 pigment for plastic and coating applications using a TiO.sub.2 slurry as a feedstock. More particularly, this invention relates to a slurry process for preparing a silanized TiO.sub.2 pigment for plastic and coating applications.
TiO.sub.2 is generally produced by hydrolyzing an aqueous solution of titanium sulfate and calcining the hydrolyzate at 750.degree. to 1000.degree. C., or oxidizing titanium tetrachloride at elevated temperatures followed by cooling to temperatures below 600.degree. C. A resulting slurry is formed by the addition of water to the TiO.sub.2 pigment. Certain finishing steps are required to be performed on the resulting slurries such as filtering, washing, drying and fluid energy milling. A fluid energy mill, also referred to as a micronizer, involves introducing dry TiO.sub.2 pigment and a fluid, e.g., air or steam into the outer portion of an inwardly spiraling vortex so as to convey the TiO.sub.2 at high velocity against the housing of the spiral vortex to fracture agglomerates as described in U.S. Pat. No. 4,427,45 1. Nonetheless, such milling is expensive and energy intensive. Fluid energy milling requires high energy input and intensive agitation.
U.S. Pat. No. 3,834,924 discloses surface modified pigments produced by adding amino organosilane to a pigment dispersion directly in a suitable solids mixing apparatus. The process therein eliminates the need for filtration, however, conventional finishing steps such as fluid energy milling are not mentioned.
U.S. Pat. No. 4,061,503 and U.S. Pat. No. 4,151,154 describe enhanced dispersibility of TiO.sub.2 in paints and plastics. Therein, the TiO.sub.2 is surface treated with a silane possessing at least two hydrolyzable groups bonded to silicon and an organic group containing a polyalkylene oxide group. In a U.S. Pat. No. 4,061,503 Example, the use of silane reduced the grinding time of TiO.sub.2 pigment for use in a water reducible enamel and post treatment of silane to TiO.sub.2. Each of the prior art processes suffers from various deficiencies and further improvements are desirable.
This invention provides a step forward in preparing TiO.sub.2 for pigmenting plastics and coatings. The present invention provides numerous advantages. Mechanical handling of the high solids slurry is made easier by lowering its viscosity, and eliminating the need for fluid energy milling results in a more economical, less energy intensive process. In addition, media milling eliminates the need for an additional step after drying to reduce the particle size further.